Abstract: We examined the variability of three ultra-luminous X-ray sources ULXs inthe 2003, 110 ks XMM-Newton observation of NGC253. Remarkably, we discoveredULX1 to be three times more variable than ULX2 in the 0.3-10 keV band, eventhough ULX2 is brighter. Indeed, ULX1 exhibits a power density spectrum that isconsistent with the canonical high state or very high-steep power law state,but not the canonical low state. The 0.3-10 keV emission of ULX1 ispredominantly non-thermal, and may be related to the very high state. We alsofitted the ULX spectra with disc blackbody, slim disc and convolutionComptonization SIMPL x DISKBB models. The brightest ULX spectra are usuallydescribed by a two emission components disc blackbody + Comptonizedcomponent; however, the SIMPL model results in a single emission component,and may help determine whether the well known soft excess is a feature of ULXspectra or an artifact of the two-component model. The SIMPL models wererejected for ULX3 and also for the black hole + Wolf-Rayet binary IC10 X-1;hence, we infer that the observed soft-excesses are genuine features of ULXemission spectra. We use an extended corona scenario to explain the soft excessseen in all the highest quality ULX spectra, and provide a mechanism forstellar mass black holes to exhibit super-Eddington luminosities whileremaining locally sub-Eddington.